Trona is a mineral found in large underground deposits near Green River, Wyo., and contains around 90% sodium sesquicarbonate, Na.sub.2 CO.sub.3.NaHCO.sub.3.2H.sub.2 O and the remainder of waste materials including marlstone, oil shale, shortite and organic impurities in addition to a small amount of free water. The sodium sesquicarbonate content of the trona is commerically important in the manufacture of soda ash (sodium carbonate).
One process for the manufacture of soda ash from trona is the so-called "Monohydrate Process". In this process, the mined trona ore is crushed into small pieces, which are calcined to impure soda ash. The calcining step removes some of the organic matter naturally found in the ore. The impure soda ash is then dissolved in a dissolver unit using natural river water and recycled condensate. The dissolver contains insolubles in addition to the sodium carbonate in solution. The material from the dissolver is transferred to a rake classifier where substantially all of the large pieces of insoluble material is removed. The solution coming from the rake classifier is conveyed to a settler (thickener) for removal of fine insolubles. The overflow from the thickener is treated with activated carbon to remove substantially all the undesirable dissolved organic impurities. Normally the activated carbon-treated solution is filtered in a leaf-type pressure filter to separate the spent activated carbon. The stream from the pressure filter is passed through a polish filter to remove small amounts of suspended activated carbon. The clear filtered solution of sodium carbonate is conveyed to an evaporator for removal of water, resulting in the crystallizing of sodium carbonate monohydrate. Sodium carbonate monohydrate crystals are separated from mother liquor and dried in a calciner to produce high grade sodium carbonate. The mother liquor is recycled to the evaporator or purged to containment ponds.
Processes of this type are disclosed in many publications, including U.S. Pat. Nos. 2,343,080; 2,343,081; 2,770,524; 2,962,348; 3,131,996; 3,260,567; and 3,528,766.
Organic impurities are naturally present in the trona and dissolve in the process sodium carbonate solution. During continuous plant production of soda ash the concentration of these organic impurities builds up in the crystallizer. When the concentration reaches a certain critical level, unexceptable foaming occurs in the crystallizer. The concentration of the dissolved organic impurities in the crystallizer can be controlled by several methods.
Previously, several techniques have been used to reduce the concentration of soluble organics in the evaporator. One method that has been used is to purge the crystallizer of small amounts of saturated sodium carbonate solution containing the high concetnration of dissolved organic impurities and replace it with feed solution of sodium carbonate wherein the organic impurity concentrations are very low. This method is not preferred as it creates pollution problems with the purge stream.
A second method for removing dissolved organic material is to bleed-off a stream of purge liquor from the evaporator solution and combine it with the stream of clarified and filtered solution of sodium carbonate before it is treated with an absorbent, such as activated carbon. Generally, this method is an effective means of reducing the concentration of soluble organic impurities, but large amounts of activated carbon are required to perform this double treatment.
A variety of anti-foaming agents are added to the evaporator solution to control foam. This method is operative but creates an added expense and will not prevent foaming when the concentration of dissolved organic impurities becomes very high.